1. Field of the Invention
This invention relates to a dry toner used in recording processes that utilize electrophotography, electrostatic recording, magnetic recording and toner-jet recording, and an image forming method which employs such a dry toner. More particularly, the present invention relates to a toner used in image-forming apparatus utilizable in copying machines, printers, facsimile machines and plotters, and an image forming method which employs such a toner. The present invention also provides a process for producing the toner.
2. Related Background Art
A number of methods are conventionally known as electrophotography as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 42-23910 and 43-24748 and so forth. In general, copied images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a dry toner (hereinafter call to xe2x80x9ca tonerxe2x80x9d) to form a toner image, transferring the toner image to a transfer medium such as paper or film, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor.
As methods by which the electrostatic latent image is rendered visible, developing methods such as cascade development, magnetic brush development and pressure development are known in the art. Another method is also known in which, using a magnetic toner and using a rotary sleeve provided with a magnet at the core, the magnetic toner is caused to fly across the sleeve and a photosensitive member by the aid of an electric field.
One-component development systems require no carrier such as glass beads or iron powder required in two-component development systems, and hence can make developing assemblies themselves small-sized and light-weight. Also, since in the two-component development systems the concentration of toner in carrier must be kept constant, a device for detecting toner concentration so as to supply the toner in the desired quantity is required, resulting in a large size and weight for the developing assemblies. In the one-component development system, such a device is not required, and hence the developing assemblies can commonly be made relatively light-weight.
As printers, LED printers or LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. More specifically, those which hitherto have a resolution of 300 or 600 dpi are being replaced by those having a resolution of 1,200 or 2,400 dpi. Accordingly, with such a trend, the developing systems are now required to achieve a high minuteness. Copying machines have also made progress to have higher functions, and hence they trend toward digital systems. In this trend, chiefly employed is a method in which electrostatic latent images are formed by using a laser. Hence, the copying machines also trend toward a high resolution and, like the printers, it has been sought to provide a developing system with higher resolution and higher minuteness.
In order to achieve such higher resolution and higher minuteness, it is required to make toners have a smaller particle diameter. However, making toners have a smaller particle diameter results in a great scattering of the chargeability of toner particles, and how to control it becomes important in order to achieve such an object.
For example, Japanese Patent Application Laid-Open No. 4-276762 discloses a proposal of a toner comprising toner particles produced by polymerization and having an average particle diameter of 3 to 8 xcexcm to the surfaces of which specific carbon black has been made to adhere. When images are reproduced using such a toner provided on particle surfaces with a material capable of controlling chargeability, the carbon black kept adhering to particle surfaces may, e.g., come off upon paper feed of about 5,000 sheets or more to cause a great variation of charging performance of toner, resulting in an insufficient charging stability.
A proposal for imparting charging stability from the viewpoint of materials is also disclosed in, e.g., Japanese Patent Application Laid-Open No. 6-242631. However, as a result of image evaluation actually made by a method disclosed in this publication, there has proved to be room for further improvement in respect of resolution.
In order to make the charging performance of toner uniform, a method is also employed in which a toner on a toner-carrying member is regulated by a strong force by means of a toner thickness regulation member. Where the toner is regulated by a strong force, the toner tends to deteriorate because of friction to tend to cause a lowering of image quality as images are reproduced on a larger number of sheets.
Accordingly, it becomes necessary to enhance the strength of toner. This can commonly be solved by making binder resin of toner have a higher glass transition temperature or introducing a cross-linking component into the binder resin to enhance modulus of elasticity in the region of temperature not higher than the glass transition temperature of the toner. As the result, however, the fixing temperature at the time of image formation must be set high, or, in the case of heat roller fixing, the pressure applied to the roller must be set a little high. This may cause difficulties such that power consumption increases in accordance with necessary heat energy and roller contamination and wind-around offset may occur very frequently.
In order to lower the fixing temperature on the other hand, a method is available in which the binder resin is made to have a low glass transition temperature or have less cross-linking component. Such a method, however, is not preferable because it may cause the difficulties as stated above or may further cause a lowering of blocking resistance during storage of toners.
Various methods and apparatus are also proposed with regard to processes by which toner images are fixed to sheets such as paper and film. At present, a method most commonly used is a pressure heating method using a heating roller or using a stationary heat-generating heater through a heat-resistant film.
The pressure heating method using a heating roller is a method in which the toner image surface of a fixing target sheet is brought into pressure contact with the surface of a heating roller having a releasability to the toner and the fixing target sheet is passed therethrough under pressure contact. In this method, the heating-roller surface and the toner image on the fixing target sheet come into contact under application of a pressure, and hence a very good thermal efficiency can be achieved when the toner image is fixed onto the fixing target sheet, and rapid fixing can be effected.
In this fixing method, however, the heating-roller surface and the toner image come into contact in a molten state under application of a pressure, and hence part of the toner image may adhere and transfer to the fixing-roller surface, which may again transfer to the subsequent fixing target sheet to tend to contaminate the fixing target sheet, which is what is called an offset phenomenon. Such an offset phenomenon is greatly affected by fixing speed and fixing temperature. Accordingly, it is commonly attempted to set the surface temperature of the fixing roller relatively low in the case of fixing at a low speed and set the surface temperature of the fixing roller relatively high in the case of fixing at a high speed so that the quantity of heat imparted to the toner from the heating roller to fix the toner image can always be controlled at a constant level to keep the offset phenomenon from occurring.
The toner on the fixing target sheet is formed in some layers as toner layers. Hence, especially in a system where the fixing is at a high speed and the fixing roller has a high surface temperature, the uppermost toner layer coming in contact with the heating roller and the lowermost toner layer coming in contact with the fixing target sheet have a great difference in temperature. Hence, when the heating roller has a high surface temperature, the toner at the uppermost layer tends to cause the offset phenomenon (i.e., high-temperature offset), and, when the heating roller has a low surface temperature, the toner at the lowermost layer does not melt sufficiently to tend to cause a phenomenon where the toner does not fix to the fixing target sheet (i.e., low-temperature offset).
As a method for solving such a problem, usually employed in the case of fixing at a high speed is a method in which pressure is set higher at the time of fixing to anchor the toner to the fixing target sheet. This method can make the fixing roller temperature low to a certain extent and enables prevention of the high-temperature offset phenomenon of the uppermost layer. However, since a very great shear force is applied to the toner, the fixing target sheet tends to wind around the fixing roller to cause wind-around offset, or separation marks of separating claws for separating the fixing target sheet from the fixing roller tend to appear on fixed images. Moreover, under existing circumstance, because of a high pressure, fixed images tend to cause image quality deterioration such that line images are crushed at the time of fixing or toner scatters.
In the case of high-speed fixing, it is common to use a toner having a lower melt viscosity, set the heating roller at a lower surface temperature and carry out fixing at a lower pressure than in the case of low-speed fixing to fix toner images while preventing high-temperature offset or wind-around offset from occurring. However, use of such a toner having a low melt viscosity tends to cause the offset phenomenon at high temperature.
Making toners have smaller particle diameter brings about an improvement in resolution or sharpness of images on the one hand, and fixing performance for halftone images formed by toners with small particles diameter lowers on the other hand. This phenomenon is remarkable especially in the high-speed fixing. This is because the toner is laid on halftone areas in a small quantity and the toner transferred to concave areas of the fixing target sheet receives heat in a small quantity from the heating roller, and also because fixing pressure is not well applied to the fixing target sheet at its concave areas where the pressure is blocked at its convex areas. The toner transferred to the convex areas of the fixing target sheet at the halftone areas has a small toner layer thickness, and hence the shear force applied per toner particle is greater in that areas than in solid black areas having a large toner layer thickness, so that the offset phenomenon tends to occur and fixed images with a low image quality tend to be formed.
In order to cope with recent trends toward smaller appratus, higher printing speed and networking, it is effective means to broaden anti-offset region of toners to the low-temperature side to simplify fixing assemblies or to make toners have lower fixing temperature to achieve higher-speed fixing processing.
In order to prevent the offset phenomenon, it is also possible to take means of coping with the matter by processing the heating roller surface with a material having a good releasability such as a fluorine resin or by coating the heating roller surface with a release agent such as silicone oil. However, the system where silicone oil is coated is not preferable because it not only requires a large fixing assembly, resulting in a high cost, but also has a complicated structure to tend to cause troubles.
Japanese Patent Publication No. 57-493 and Japanese Patent Applications Laid-Open Nos. 50-44836 and 57-37353 disclose methods in which resins are made asymmetric and cross-linkable to prevent the offset phenomenon, but no sufficient improvement has been made on fixing temperature.
In general, minimum fixing temperature lies between a low-temperature offset temperature and a high-temperature offset temperature, and hence serviceable temperature region lies between the minimum fixing temperature and the high-temperature offset temperature. Service fixing temperature can be made low and also the serviceable temperature region can be broadened by making the minimum fixing temperature as low as possible and making the high-temperature offset temperature (temperature at which high-temperature offset occurs) as high as possible. This enables achievement of energy saving and high-speed fixing and prevention of paper curl. Accordingly, it is always sought to provide a toner having good fixing performance and anti-offset properties.
Under such circumstances, Japanese Patent Application Laid-Open No. 9-265209 discloses that fixing temperature region can be broadened by a toner obtained using as a chief component a resin composition for toner which contains i) 100 parts by weight of a vinyl polymer composed chiefly of a low-molecular weight vinyl polymer component having a weight-average molecular weight of from 3,000 to 10,000 and a high-molecular weight vinyl polymer component having a weight-average molecular weight of from 300,000 to 1,000,000 and ii) from 0.05 to 1 part by weight of an antioxidant, and by melt-kneading this composition, followed by cooling and then fine pulverization. In this method, however, the fixing region is only shifted to the low-temperature side and there is a high possibility that the offset seriously occur on the high-temperature side.
Meanwhile, Japanese Patent Application Laid-Open No. 8-262795 discloses a proposal of a toner obtained using a binder resin comprised of a high-molecular weight styrene-acrylic resin having, in its molecular-weight distribution as measured by gel permeation chromatography, a molecular-weight peak in the region of molecular weight of 500,000 or more, a styrene-acrylic resin having a molecular-weight peak in the region of molecular weight of from 50,000 to 500,000, a styrene-acrylic resin having a cross-linked structure and a polyester resin having a molecular-weight peak in the region of molecular weight of 50,000 or less. This toner, however, is still not well adaptable to high-speed fixing.
Techniques are also proposed in which a capsule toner constituted of a core material and a shell so provided as to cover the surface of this core material is used to improve low-temperature fixing performance. Among them, it is reported that fixing can be effected by pressure only, when a low-melting point wax readily undergoing plastic deformation is used as the core material (U.S. Pat. No. 3,269,626, Japanese Patent Publications Nos. 46-15876 and 44-9880 and Japanese Patent Applications Laid-Open Nos. 48-75032 and 48-75033). However, the toner has a poor fixing roller strength and can be used only for limited purposes. Also, when a liquid material is used as the core material and where shells have a low strength, though fixable by pressure only, capsules may break in a developing assembly to contaminate the interior of a machine. Where shells have a high strength, a great pressure is required to break capsules to bring about too glossy images. Thus, it is difficult to adjust the strength of shells.
Accordingly, a capsule toner for heat roller fixing is proposed in which a resin having a low glass transition point which may cause blocking at the time of high temperature when used alone as a core material but brings about an improvement in fixing strength is used for heat-and-pressure fixing and, as shells, high-melting point resin walls have been formed by interfacial polymerization in order to impart blocking resistance (Japanese Patent Application Laid-Open No. 61-56352). However, since the wall material has a high melting point and also too tough to break easily, the performance of the core material has not completely been brought out.
Japanese Patent Application Laid-Open No. 8-286416 also discloses a technique in which an unsaturated polyester resin or a styrene-acrylic resin is adsorbed and polymerized onto toner particles obtained by suspension polymerization of a mixture of polymerizable monomers, to coat the latter with the former. This technique makes it possible to obtain a toner having very good running performance and also more improved than the above toner in respect of fixing performance, but there has been room for further improvement in respect of low-temperature fixing performance.
Capsule toners for heat roller fixing are also proposed in which under the same idea the core material has been improved in fixing strength (Japanese Patent Application Laid-Open Nos. 58-205162, 58-205163, 63-128357, 63-128358, 63-128359, 63-128360, 63-128361 and 63-128362). However, the toners are produced by spray drying and hence a burden is imposed to production equipment. Also, these take no measure for shell materials and hence the performance of the core material has not completely been brought out.
Japanese Patent Application Laid-Open No. 63-281168 also disclose a capsule toner whose shell material is a thermotropic liquid-crystal polyester, and Japanese Patent Application Laid-Open No. 4-184358 disclose a capsule toner whose shell material is a liquid-crystal polyester. In both the toners, however, the polyester is not amorphous and hence, though the resin melts sharply, energy necessary for melting is required in a large quantity and also the core material has so high a glass transition temperature as to provide a poor fixing performance.
Thus, capsule toners produced using various materials and production process are proposed, but none of them have satisfied all of sufficient low-temperature fixing performance, anti-offset properties and blocking resistance and stress resistance in developing assemblies. Especially with regard to physical properties of capsule toners that can satisfy these performances, no quantitative values have ever been elucidated.
Meanwhile, in a capsule toner disclosed in Japanese Patent Application Laid-Open No. 7-301947, specified are an extent of deformation of toner capsules under application of a load within the range of given weights in a microcompression tester and a change in degree of agglomeration before and after leaving with heating. However, with such features alone, any deterioration of toner in the process of development is not taken into account, and the toner could never be satisfactory in practice.
From the viewpoint of materials, a toner is proposed in which storage elastic modulus of a thermoplastic elastomer at 200xc2x0 C. is specified (Japanese Patent Application Laid-Open No. 7-271096). However, its effect is emphasized on the improvement in anti-offset properties and the prevention of paper from winding around the heating roller, and is still insufficient in respect of image quality.
As toners making use of general-purpose materials polyene compounds such as butadiene and isoprene, Japanese Patent Application Laid-Open No. 7-271096 discloses an example. Its effect, however, is only to improve anti-offset properties and prevent paper from winding around the heating roller.
For the reasons as stated above, although it is sought to provide in fixing processes a toner having a broad fixing temperature region and superior anti-offset properties, any toners that satisfy these points well and also satisfy high resolution and high minuteness and good charging stability are not available under existing circumstances.
With regard to processes for producing toners, Japanese Patent Application Laid-Open No. 11-160909 discloses a process comprising the step of subjecting a polymerizable monomer composition to suspension polymerization in the presence of an oil-soluble polymerization initiator until polymerization conversion comes to be in the range of from 30 to 97%, followed by addition of a water-soluble polymerization initiator to effect further polymerization. In this process, however, a macromonomer is used, which is not comparable to usual monomers in respect of reactivity, thus the process is unsatisfactory for obtaining toners having good running performance.
Meanwhile, where toner images formed on a photosensitive member in the step of development are transferred to a transfer medium in the step of transfer and in that course a transfer residual toner remains on the photosensitive member, it is necessary for the transfer residual toner to be removed in the step of cleaning and be stored in a waste toner container. In the cleaning step, blade cleaning, fur-brush cleaning, roller cleaning and so forth are conventionally used. In any methods, the transfer residual toner is mechanically scraped off or blocked up with a suitable member and collected into the waste toner container. Accordingly, problems arise because of the fact that such a member is brought into contact with the photosensitive member surface. For example, when a toner that may remain in a large quantity after transfer is used, it is necessary for the member to be strongly brought into contact with the photosensitive member surface, so that the photosensitive member wears to have a short lifetime.
From the viewpoint of apparatus, equipment of such a cleaning assembly necessarily makes apparatus large in size, providing an obstacle to an aim at making apparatus compact. Moreover, in a sense of effective utilization of toners from the viewpoint of ecology, it is desirous to use a system that produces no waste toner.
Here, techniques concerned with xe2x80x9ccleanerlessxe2x80x9d are disclosed in Japanese Patent Applications Laid-Open Nos. 59-133573, 62-203182, 63-133179, 64-20587, 2-302772, 5-2289, 5-53482, 5-61383 and so forth. These, however, do not refer to any desired constitution of toner.
Incidentally, in cleaning-at-development construction having substantially no cleaning assembly, the photosensitive member surface is rubbed with a toner and a toner-carrying member to collect toner present at non-image areas by means of the toner-carrying member and to develop image areas by means of the toner. Such construction is required. At the time of this rubbing, reverse-charged toner called transfer residual toner or fogging toner can electrically be collected with ease if its polarity can readily be reversed. For that end, it can be one means to add a polar component to the toner.
Namely, a common phenomenon is utilized such that most toners containing polar components are speedily chargeable. However, when a release agent having little polarity such as polyethylene or polypropylene is added in order to improve anti-offset properties of toner, the toner can be less speedily chargeable to inhibit smooth collection of toner on the photosensitive member in the developing step. As the result, formed are printed images whose regions having no image originally are imagewise contaminated by toner, i.e., what is called ghost images.
Thus, in the cleaning-at-development construction, a technique is sought which can achieve both fixing performance of toner and image charcteristics. Also, the technique to collect toner at the developing step can be said to be very important also in a system where members are weakly pressed against the photosensitive member for making it have a long lifetime.
An object of the present invention is to provide a dry toner having solved the problems discussed above.
Another object of the present invention is to provide a dry toner having a superior charging stability.
Still another object of the present invention is to provide a dry toner that can obtain highly minute images in a high resolution and has superior fixing performance and anti-offset properties.
A further object of the present invention is to provide a toner that can enjoy stable formation of high-quality images over a long period of time.
A still further object of the present invention is to provide a dry toner that enables its high-grade application in electrophotographic processes without adversely affecting photosensitive members, toner-carrying members and also even intermediate transfer members, and to provide a process for producing such a dry toner and an image-forming method making use of such a dry toner.
A still further object of the present invention is to provide an image-forming method for electrophotography by which, in a contact development type image-forming process having cleanerless construction or making use of an intermediate transfer member, resolution and transfer performance can be improved while maintaining fixing performance, any ghost images can be kept from occurring on account of an improvement in toner collection performance, and also even running performance can greatly be improved.
To achieve the above objects, the present invention provides a dry toner comprising toner particles containing at least a binder resin, a colorant and a wax component, and an external additive, wherein;
(1) the binder resin contains a component derived from a monomer selected from the group consisting of butadiene, isoprene and chloroprene;
(2) the toner has a main glass transition temperature (Tg) of from 40xc2x0 C. to 70xc2x0 C. as measured by differential scanning calorimetry (DSC);
(3) where specific surface area measured by the BET method when the toner is left for 72 hours in an environment of 23xc2x0 C. atmospheric temperature and 65% relative humidity is represented by A (m2/g) and specific surface area measured by the BET method when the toner is left for 72 hours in an environment of 50xc2x0 C. atmospheric temperature and 3% relative humidity is represented by B (m2/g), the toner satisfies the following relationship:
0.8xe2x89xa6Axe2x89xa64.0, 0.80xe2x89xa6(B/A)xe2x89xa61.05;
(4) in a toner""s number-based circle-corresponding diameter/circularity scatter diagram as measured with a flow type particle image analyzer, the toner has a circle-corresponding number-average particle diameter D1 of from 2 xcexcm to 10 xcexcm and has an average circularity of from 0.950 to 0.995 and a circularity standard deviation of less than 0.040; and
(5) the toner has, in its molecular-weight distribution of tetrahydrofuran(THF)-soluble matter as measured by gel permeation chromatography (GPC), a main-peak molecular weight in the region of from 2,000 to 100,000 and contains a THF-insoluble matter in an amount of from 5% by weight to 60% by weight.
The present invention also provides a process for producing a dry toner, comprising dispersing a polymerizable monomer composition in an aqueous medium to effect granulation, followed by polymerization in the aqueous medium to form toner particles; the polymerizable monomer composition comprising a binder resin which contains a component derived from a monomer selected from the group consisting of butadiene, isoprene and chloroprene, a polymerizable vinyl monomer, a colorant, a wax and a polymerization initiator;
the polymerization initiator being a radical polymerization initiator, and a radical polymerization initiator being further added when the conversion of polymerization reaction is in the range of from 10% by weight to 95% by weight.
The present invention still also provides an image forming method comprising;
a charging step of applying a voltage to a charging member to charge an electrostatic latent image bearing member;
an electrostatic latent image forming step of forming an electrostatic latent image on the electrostatic latent image bearing member thus charged;
a developing step of bringing a toner carried on a toner-carrying member into adhesion to the electrostatic latent image formed on the electrostatic latent image bearing member, to form a toner image on the electrostatic latent image bearing member;
a transfer step of electrostatically transferring the toner image formed on the electrostatic latent image bearing member, to a transfer medium via, or not via, an intermediate transfer member; and
a fixing step of fixing the toner image transferred electrostatically to the transfer medium;
the toner being a dry toner comprising toner particles containing at least a binder resin, a colorant and a wax component, and an external additive, wherein;
(1) the binder resin contains a component derived from a monomer selected from the group consisting of butadiene, isoprene and chloroprene;
(2) the toner has a main glass transition temperature (Tg) of from 40xc2x0 C. to 70xc2x0 C. as measured by differential scanning calorimetry (DSC);
(3) where specific surface area measured by the BET method when the toner is left for 72 hours in an environment of 23xc2x0 C. atmospheric temperature and 65% relative humidity is represented by A (m2/g) and specific surface area measured by the BET method when the toner is left for 72 hours in an environment of 50xc2x0 C. atmospheric temperature and 3% relative humidity is represented by B (m2/g), the toner satisfies the following relationship:
0.8xe2x89xa6Axe2x89xa64.0, 0.80xe2x89xa6(B/A)xe2x89xa61.05;
(4) in a toner""s number-based circle-corresponding diameter/circularity scatter diagram as measured with a flow type particle image analyzer, the toner has a circle-corresponding number-average particle diameter D1 of from 2 xcexcm to 10 xcexcm and has an average circularity of from 0.950 to 0.995 and a circularity standard deviation of less than 0.040; and
(5) the toner has, in its molecular-weight distribution of tetrahydrofuran(THF)-soluble matter as measured by gel permeation chromatography (GPC), a main-peak molecular weight in the region of from 2,000 to 100,000 and contains a THF-insoluble matter in an amount of from 5% by weight to 60% by weight.